The invention relates to the field of aerial imagery acquisition by lightweight UAVs. Note that imagery refers to any kind of image acquisition using any kind of imager ranging from standard, visible spectrum cameras to narrow band or multispectral sensors.
UAVs are generally equipped with some form of automatic navigation means that allow them to sequentially navigate along predetermined paths, towards given geographic locations (or waypoints), which are defined by their GPS coordinates and altitude. In most cases, the automatic navigation means rely at least on a GPS (Global Positioning System) receiver to function.
UAVs are also often equipped with some form of automatic attitude control means that allow the control of the aircraft orientation in space (often defined by the three Euler angles: yaw, pitch and roll, see FIG. 1) and reject perturbations from wind or turbulences. The automatic attitude control means are typically used by the automatic navigation means to control the UAV trajectory. For example, if the UAV should reach a higher altitude, the automatic navigation means will instruct the automatic attitude control means to regulate a nose-up orientation, which will result in the UAV climbing. Likewise, the automatic navigation means usually achieve changes of heading by instructing the automatic attitude control means to regulate a laterally tilted, or rolled, attitude.
In the field of aerial imaging, distinction is made between images taken by a camera which objective is directed perpendicularly with respect to the ground and slanted images taken from an arbitrary perspective, i.e. with an arbitrary orientation of the camera. In order to acquire slanted images of some photographic target, UAVs typically rely on a pan-tilt apparatus or similar mechanical contraption to change the relative orientation of the imager with respect to the UAV. This apparatus comes with a cost in terms of weight and complexity, which negatively impacts the price, size and robustness of small UAVs. For this reason, a class of small UAVs exists that are equipped with imagers with a fixed orientation with respect to the UAV. These UAVs are generally able to get images in a vertical direction, or at some predefined and fixed angles, but not from arbitrarily perspectives.
In order to uniquely define a perspective on a photographic target, a set of parameters must be provided. An example of such set is:                the bearing and the altitude of the imager with respect to the photographic target        the distance between the photographic target and the camera (which, at a given focal length or zoom settings, is proportional to the area around the photographic target that will be covered by the image)        
Slanted images taken from arbitrary perspectives are useful in many cases such as:                Specific views for security purposes, for example observing or mapping building walls, bridges, monuments, etc.        Images of steep slopes for geological studies (mountains, glaciers, cliffs, etc.), or agricultural purposes as for example observing vineyards.        Openings to new purposes in architecture, housing market, civil engineering, military and spying applications, etc.        
The known method for acquiring slanted images by a UAV uses a “pan-tilt” system allowing arbitrary directing the camera relative to the aircraft. A drawback of such a system is its mechanical complexity, high weight and relative high cost. Therefore, it becomes advantageous to use a (set of) camera(s) rigidly attached to the airframe of the UAV.
Some methods exist that allow to cope with airplanes having fixed cameras. However, they always assume a globally level orientation of the plane attitude during the acquisition of images and therefore rely on pre-planning of straight lines towards waypoints. For instance the document “CECCARELLI N ET AL: “Micro UAV Path Planning for Reconnaissance in Wind”, AMERICAN CONTROL CONFERENCE, 2007. ACC'07, IEEE, PISCATAWAY, N.J., USA, 9 Jul. 2007, addresses the control of a Micro Unmanned Aerial Vehicle (MAV) for the purpose of obtaining video footage of a set of known ground targets with preferred azimuthal viewing angles, using fixed onboard cameras. In this case, the control is exercised only through the selection of classical waypoints, without interruption of the classical navigation to temporary control of the orientation or the attitude of the plane to orient the objective towards the target. In particular:                the strategy is limited to acting on the position of classical attracting waypoints by making sure in advance that the waypoints are positioned intelligently so that the fixed camera will be aiming to the target when the plane is flying straight and almost level between the waypoints        the plane is not actively tilted during the picture taking process in order to further orient the objective towards the target, therefore the proposed method will not work with camera looking straight downward,        the altitude is not expected to vary during the picture taking process,        the preferred angle defining the perspective of the image is only azimuthal, whereas the elevation is arbitrary        
The document US 2010/286859 describes the generation of a flight plan made of a plurality of waypoints such that the camera “overlaps” the target. The method comprises graphically identifying, on a map displayed on the display device, a desired target for the surveillance module. The camera will take pictures along the programmed flight plan. This method is only suitable for taking pictures while the aerial vehicle is following a flying path. The altitude or the orientation of the flight plan can be modified in order to take the picture in an adequate way.
However, none of the existing methods allows taking pictures of predefined targets by transiently orienting the plane (and therefore the camera) towards the target. They rather assume level flight towards waypoints that have been intelligently positioned in order to make sure the camera will see the targets at some point. These existing methods therefore significantly limit the kinds of perspectives one can achieve and require the entire flight planning (by positioning the waypoints) to be made accordingly to the targets to be imaged.